Numerous methods and devices for determining at least one flow property of fluid media, i.e., liquids and/or gases, are known from the related art. The flow properties as possible parameters may be arbitrary measurable physical and/or chemical properties, which qualify or quantify a flow of the fluid medium. This may be a flow rate and/or a mass flow and/or a volume flow in particular.
The present invention is described below in particular with reference to so-called hot-film mass air flow sensors, such as those known from Konrad Reif (ed.): Sensoren im Kraftfahrzeug [Sensors in the Automobile], 1st edition, 2010, pages 146-148. Such hot-film mass air flow sensors are generally based on a sensor chip, in particular a silicon sensor chip, for example, including a sensor membrane as the measuring surface or a sensor area over which the flowing fluid medium may flow. The sensor chip generally includes at least one heating element and at least two temperature sensors situated on the measuring surface of the sensor chip, for example, the one temperature sensor being situated upstream from the heating element and the other temperature sensor being situated downstream from the heating element. A mass flow and/or volume flow of the fluid medium may be inferred from an asymmetry of the temperature profile detected by the temperature sensors.
Hot-film mass air flow sensors are usually configured as plug-in sensors, which are insertable permanently or replaceably into a flow tube. For example, this flow tube may be an intake tract of an internal combustion engine.
A substream of the medium flows through at least one main channel provided in the hot-film mass air flow sensor. A bypass channel is formed between the inlet and the outlet of the main channel. In particular, the bypass channel is configured in such a way that it has a curved section for deflecting the substream of the medium entering through the inlet of the main channel, the curved section merges in the further course into a section in which the sensor chip is situated. The section mentioned last is the actual measuring channel, in which the sensor chip is situated.
Such hot-film mass air flow sensors must in practice meet numerous requirements. In addition to the objective of reducing the pressure drop on the hot-film mass air flow sensor on the whole through suitable fluidic designs, one of the main challenges is to further improve the signal quality and the robustness of the devices with respect to contamination by oil and water droplets as well as soot, dust and other solid particles. This signal quality relates to a mass flow of the medium through the measuring channel leading to the sensor chip, for example, and optionally also relates to a reduction of a signal drift and an improvement in the signal-to-noise ratio. The signal drift relates to the deviation in mass flow of the medium, for example, in the sense of a change in the characteristic line relationship between the mass flow actually occurring and the signal to be output, which is ascertained within the scope of the calibration during the manufacture. During the ascertainment of the signal-to-noise ratio, the sensor signals output in rapid chronological sequence are taken into account, whereas the characteristic line drift or the signal drift relates to a change in the average.
In the case of traditional hot-film mass air flow sensors of the type described here, a sensor carrier including a sensor chip mounted thereon or introduced therein generally protrudes into the measuring channel. For example, the sensor chip may be glued onto or into the sensor carrier. The sensor carrier may form a unit together with a bottom plate made of metal, on which an electronic system, a control circuit and evaluation circuit (for example, including a circuit carrier, in particular a circuit board) may be glued. For example, the sensor carrier may be configured as an injection-molded plastic part of an electronic module. The sensor chip and the control circuit and evaluation circuit may be interconnected by bond connections, for example. The electronic module created in this way may be glued into a sensor housing, for example, and the entire plug-in sensor may then be closed with covers.
Patent document DE 10 2011 005 768 A1 discusses a device for detecting at least one property of a fluid medium including at least one sensor housing which is introducible into the fluid medium. The sensor housing has at least one channel through which the fluid medium may flow and which has at least one outlet opening. The fluid medium may flow out through the outlet opening after flowing through the channel. The sensor housing has at least one housing body and at least one cover. The outlet opening is situated in the cover. The housing body has at least one collar section engaging in the outlet opening and forming at least a section of an edge of the outlet opening.
Despite the numerous advantages of the devices known from the related art, they still retain potential for improvement with respect to functional aspects. Thus such devices usually have an electronic module which is accommodated in an electronics space of the sensor housing. The electronic module, the remaining area of the sensor housing, in which the channel is formed, and the cover are constructed in such a way that the one part of the sensor carrier including the sensor chip is accommodated in the area of the channel around which the flow circulates. The other part including the electronics is situated in the electronics space of the sensor housing so that it is protected from external effects. The areas are separated by an adhesive tape on the cover, which is located above the center of the sensor chip. The fit of the cover and the housing body of the sensor housing is sometimes not unambiguously defined. The actual dimensions of the cover and of the housing body vary since the cover and the housing body usually originate from injection molding tools having multiple cavities. Consequently, the resulting position of the cover relative to the housing body is subject to fluctuations. The position tolerances of the cover in turn result in a variance in the characteristic line behavior, which must be compensated for in the equalizing process.